During well operations, chemicals are often injected into the underlying formation in a process known as stimulation. Stimulation of the formation is one technique that can be used to increase the net permeability of a formation or reservoir. Some exemplary known stimulation techniques include: (1) injection of chemicals into the wellbore to react with and dissolve conditions limiting production (e.g., the presence of clays and drilling solids); (2) injection of chemicals through the wellbore and into the formation to react with and dissolve portions of the formation, or to create alternative flow paths for recoverable hydrocarbons (e.g. acid-fracturing or matrix-acidizing); and (3) injection of chemicals through the wellbore and into the formation at pressures sufficient to cause fractures in the formation, thereby creating a flow channels through which hydrocarbons can more readily move from the formation into the wellbore.
In particular, sandstone formations are particularly susceptible to formation damage from formation minerals such as clay and other siliceous deposits. Historically, acid, or acid-based fluids have been used in the treatment or stimulation due to their ability to dissolve both formation minerals and contaminants (e.g., drilling fluid coating the wellbore or that has penetrated into the formation) introduced into the wellbore/formation during drilling or remedial operations.
The removal of formation minerals and other deposits, such as clays or drilling solids, are key concerns in well completion operations. The known prior art techniques noted above typically involve highly concentrated acids, such as hydrofluoric acid (HF) and hydrochloric-hydrofluoric acid mixtures, which are both highly corrosive and can create dangerous operating conditions.
One difficulty encountered with traditional sandstone stimulation operations that employ HF is that HF can precipitate into the formation, causing formation damage and limiting matrix stimulation treatment efficiency. The damage caused by the fluoride precipitations are aggravated by higher temperatures. Another difficulty encountered with traditional HF sandstone stimulation is the acid reacts instantaneously with the formation, thereby limiting penetration of the acid into the formation. Therefore, it would be advantageous to use a stimulation fluid that does not form precipitants when it reacts with the sandstone formation at high temperatures and has a slower reaction rate.